How do you keep chicken warm in a chicken tractor during winter?

Try googling something like backyard poultry. You'll end up with a company in the UK that has small chicken houses with runs. Should give you an idea of what you need to make for the number of chickens you have. They don't need much, but they will need more than just a cage.

You'll have to use the garage until a coop of some sort can be had. A chicken tractor is not enough shelter for our Michigan winters. They need some sort of draft free housing. Even then, frostbite can be a problem when the temps get real cold. Trust me, you don't want to deal with that! An old garden or storage shed would work. It would be best if it had windows. They will poop on everything and try to get up high, so you might want to section off an area just for them in the garage until you can get suitable shelter.

If money is tight you can cover the open (wire) areas with heavy plastic but be sure to have a low and high vent (about 3x12") to allow air flow and to expel excess heat during the day (greenhouse effect). Be sure to keep the lower vent from being blocked by snow.

I've always enclosed the smaller runs with double layer of plastic and inflated them with a greenhouse inflation fan. This keeps the wind noise down and adds strength to the plastic if some predator tries to go through it (hard for them to get a hold of the plastic).

Use a 60 to 100W light bulb in the coop, be sure to use a porcelain fixture and if you can put it on a thermostat that will turn it on only if it gets below 34F and turn off at 38F that will prevent overheating.

Increase carbs in their diet with Wheat/Oats/rye or corn. This will give them extra fat reserves to help them through the colder spells. Keep and eye on their weight so they don't get too fat. Any change in diet should be a gradual process.